It is possible to have air hunger secondary to diabetes when the blood becomes acidotic. This is unlikely.
I would suggest a pulmonary consult. They will give you a chemical challenge while will be definitive. It is possible you have an asthmatic problem. These tend to worsen when swimming because droplets of water enter the trachea, producing mild spasms within the passageways within the lungs.
There are lots of reasons for experiencing chest pain.
It can be from muscle problems, infection and especially if you have a fever and a cough or just feel unwell, heart problems, anxiety and stress, stomach problems, lung infections.
Having diabetes should have no bearing on your chest to feel like it is under pressure.
The only way to find out is to make an appointment and get examined by your doctor for you to get the correct diagnosis and the appropriate treatment.
Make an appointment as soon as you can so that you can be treated appropriately.
Best wishes.
Chest pain could be of something else. Get a good consultant with your doctor.